1. Field of the Invention
The present invention relates to a semiconductor photoreceiving device that is suitable for use in optical communications and performs opto-electric conversion of light at high speeds, and more particularly to a semiconductor photoreceiving device that can selectively extract a signal of long wavelength light in multiplexed optical communications (especially with two wavelengths).
This application is a counterpart application of Japanese application Serial Number 212945/2000, filed Jul. 13, 2000, the subject matter of which is incorporated herein by reference.
2. Description of Related Art
In general, light of wavelengths 1.3 xcexcm and 1.55 xcexcm is used as the optical signal in optical communications, especially flat mounted optical modules. In optical integrated circuits, this 1.3 xcexcm and 1.55 xcexcm optical signal is present as multiplexed light. Accordingly, it is necessary that the photoreceiving device be an element that can selectively receive both 1.3 xcexcm optical signals and 1.55 xcexcm optical signals.
Conventionally, the photoreceiving devices that selectively receive optical signals with wavelengths of 1.3 xcexcm (hereinafter referred to as short wavelength) are provided with a light-absorbing layer composed of InGaAsP with a band gap wavelength of about 1.4 xcexcm. However, the light-absorbing layer generally has the property of absorbing light with wavelengths shorter than the band gap wavelength of the light-absorbing layer. Accordingly, it is very difficult to form a light-absorbing layer that can receive only an optical signal of 1.55 xcexcm (hereinafter referred to as long wavelength) in a single layer.
Consequently, the method of interposing an optical filter between the multiplexed light entering the photoreceiving device and the photoreceiving surface of the photoreceiving device has been used for the selective reception of long wavelength optical signals, as disclosed in the Reference 1 (Ishigami et al., xe2x80x9cAutomatic Machine For Bonding Fiber Block To Planar Lightwave Circuitxe2x80x9d, 1998 Institute of Electronics, Information and Communication Engineers, Electronics Society Conference C-3-87), Reference 2 (Hashimoto et al., xe2x80x9c1.3/1.55 xcexcm WDM optical module for full duplex operation using PLC platformxe2x80x9d, 1998 Institute of Electronics, Information and Communication Engineers, Electronics Society Conference C-3-110), and Reference 3 (Maeda et al., xe2x80x9cWDM by multilayered dielectric filter on silica based waveguide (3)xe2x80x9d, 1998 Institute of Electronics, Information and Communication Engineers, Electronics Society Conference C-3-150).
As another method for the selective reception of long wavelength optical signals, the inventors of the present invention previously proposed a semiconductor photoreceiving device disclosed in Reference 4 (Japanese Unexamined Patent Pubication No. 2000-77702). The semiconductor photoreceiving device in Reference 4 comprises a first light-absorbing layer and a second light-absorbing layer in that order within an optical path for multiplexed light; wherein this first light-absorbing layer is composed of a material with a band gap wavelength longer than 1.3 xcexcm and shorter than 1.55 xcexcm (for example, InGaAsP), and the second light-absorbing layer is composed of a material having a band gap wavelength longer than 1.55 xcexcm (for example, InGaAs). Shorter wavelength light is thereby absorbed by the first light-absorbing layer and long wavelength light that passes through this first light-absorbing layer is absorbed by the second light-absorbing layer.
However, in the case of using an optical filter with the object of selectively receiving long wavelength light, it becomes necessary to perform a process of making a recess on the waveguide of the optical integrated circuit (PLC: planar lightwave circuit) in order to insert the optical filter. This results in a high insertion loss for the optical waveguide, increases PLC processing costs, and results in relatively high part costs for the optical filter.
In a photoreceiving device comprising a first light-absorbing layer and a second light-absorbing layer, the first light-absorbing layer must be thick so that the selection ratio of short wavelength light to long wavelength light is the desirable selection ratio for the photoreceiving device. As a result, there is high stress between the first light-absorbing layer and the layers formed on top of the first light-absorbing layer, and there is a risk of strain occurring in each layer formed on top of the first light-absorbing layer. Also, the first light-absorbing layer is formed by epitaxial growth; the cost of the photoreceiving device increases in proportion to the thickness of the epitaxial layer.
It is an object of the present invention to provide a photoreceiving device used in multiplexed optical communications including short wavelength light and long wavelength light for selectively receiving long wavelength light, that is inexpensive and has good properties, without cutting away the PLC waveguide and inserting an optical filter.
In order to achieve this object, the present invention is to provide a semiconductor photoreceiving device having the following constitution for selectively receiving long wavelength light from multiplexed light including long wavelength light and short wavelength light. Specifically, this semiconductor photoreceiving device comprises a multilayered film of alternately stacked layers of materials having mutually different indexes of refraction. The thickness of each layer and the number of layers in the multilayered film are designed such that long wavelength light is transmitted and short wavelength light is reflected by the multilayered film. The photoreceiving device further comprises a first light-absorbing layer that is composed of a material having a band gap wavelength longer than the long wavelength light. Also, the photoreceiving device has a structure such that multiplexed light enters the first light-absorbing layer through the multilayered film.
When multiplexed light comprising long wavelength light and short wavelength light enters this multilayered film, the short wavelength light is reflected by the multilayered film and the long wavelength light passes through the multilayered film and reaches the light-absorbing layer. The long wavelength light can thereby be selectively absorbed, or received, by the light-absorbing layer. Therefore, it is unnecessary to insert a conventional optical filter requiring a cutting operation, between the photoreceiving device and the incoming multiplexed light. Consequently, problems arising from establishing the optical filter (PLC processing costs, optical filter costs, optical waveguide insertion loss) are avoided.
This type of semiconductor photoreceiving device preferably comprises a substrate having a first main surface and a second main surface; the first light-absorbing layer may be established on the first main surface side of the substrate and the multilayered film may be established on the second main surface side of the substrate.
When the photoreceiving surface for the multiplexed light is assumed to be the second main surface side of the substrate, and the first main surface of the substrate is referred to as the top surface, while the second main surface is referred to as the back surface, the semiconductor photoreceiving device having this type of constitution is referred to as a back surface incidence (or entry) type photoreceiving device.
The semiconductor photoreceiving device may also be constituted such that the substrate has a first main surface and the multilayered film is established on the first main surface side of the substrate with the first light-absorbing layer interposed therebetween.
A photoreceiving device with such a constitution is referred to as a top surface incidence (or entry) type photoreceiving device in contrast to the abovementioned back surface incidence type photoreceiving device.
In these back surface incidence type and top surface incidence type photoreceiving devices, multiplexed light including long wavelength light and short wavelength light first enters the multilayered film and then the short wavelength light is reflected by this multilayered film. After that, the long wavelength light passes through the multilayered film, passes through the substrate, reaches the first light-absorbing layer, and is absorbed (received) by this first light-absorbing layer.
The top surface incidence type and back surface incidence type photoreceiving devices are suitable for use in the case of incident light spread over a broad range, such as in monitor photodiodes (MPD), for example, because the photoreceiving surface can be made as large as the chip size.
The constitution of the semiconductor photoreceiving device is preferably provided a substrate having a first main surface and a side surface. The first light-absorbing layer may be established on the first main surface side of the substrate and the multilayered film may be established on the side surface of the substrate. Furthermore, a mesa surface is preferably established on the substrate so that light passing through the multilayered film is reflected towards the first light-absorbing layer.
A photoreceiving device with such a constitution is called the end surface incidence (or entry) type. The end surface incidence type device is able to operate at higher speeds than the top surface or back surface incidence type devices. Because a mesa surface is established on the substrate, light passing through the multilayered film and entering (or impinging on or arriving at) the substrate is reflected by the mesa surface and this reflected light enters the second light-absorbing layer from the first main surface of the substrate. The short wavelength light in the multiplexed light is thereby reflected by the multilayered film. After this, only the long wavelength light is absorbed by the first light-absorbing layer.
The semiconductor photoreceiving devices according to the present invention may also be provided with a substrate having a first main surface, and may be constituted so that the first light-absorbing layer is established on this first main surface side and the multilayered film is disposed so that the light passing through the multilayered film is refracted and is incident on or enters the first light-absorbing layer. This type of constitution can be given as a variation of the end surface incidence type discussed above. A V-shaped groove is formed in the second main surface of the substrate by wet etching and an end surface comprising the mesa surface of the V-shaped groove is formed by cleavage or the like (for example, scribing, dicing, etching) near the peak of this V-shaped groove. This end surface becomes the photoreceiving surface and the multilayered film is established thereon. The short wavelength light in the multiplexed light entering the multilayered film is thereby reflected by the multilayered film. The light passing through the multilayered film is refracted by the end surface and passes through the substrate to reach the first light-absorbing layer. After this, the long wavelength light is absorbed by the first light-absorbing layer.
A semiconductor photoreceiving device having the constitution discussed above preferably further comprises a second light-absorbing layer for selectively absorbing short wavelength light from the multiplexed light. This second light-absorbing layer may be established at a position such that the light passing through the multilayered film enters the first light-absorbing layer through the second light-absorbing layer.
The second light-absorbing layer is preferably composed of a material having a band gap wavelength longer than the short wavelength light and shorter than the long wavelength light.
It is necessary to increase the thickness of the multilayered film in order to have a selection ratio of short wavelength light to long wavelength light of about xe2x88x9230 dB, For example, in order to further improve the properties of the photoreceiving device. However, when the multilayered film becomes too thick, strain occurs due to the stress on the film. As a result, there is a risk of the film separating or its properties deteriorating, causing increased current leakage or a reduced transmission rate. A multilayered film therefore should be formed into a thickness such that there is no such risk of its properties deteriorating and a second light-absorbing layer is established in the light path through which the light passing through the multilayered film reaches the first light-absorbing layer. This second light-absorbing layer has a band gap wavelength longer than the short wavelength light and shorter than the long wavelength light, and therefore the short wavelength light that is not reflected by the multilayered film can be absorbed by this layer. Meanwhile, the long wavelength light passes through this second light-absorbing layer and reaches the first light-absorbing layer.
Such a photoreceiving device can be caused to selectively intercept long wavelength light at the preferred intensity selection ratio by the actions of both the multilayered film and the second light-absorbing film that selectively absorbs short wavelength light. Accordingly, it is not necessary for either the multilayered film or the second light-absorbing layer to be thick. The risk of the strain of each layer is thereby eliminated and cost reductions for the photoreceiving device can be achieved.